If you have a good finish on a single axis move but not a two or more axis move, then perhaps the Servo Loop gain and Axis balance are out of adjustment. Also suspect could be the Bellville washer stack retaining the tool into the taper may be weak.
Love to give you my opinion, do you have transition lines in your contour,
if you can not process the info fast enough G8's will only make the problem worse. Could be any thing from Gibbs, and straps needing adjusting to backlash both are interconected. To your spindle bearings going out.
Let me know.